Super squirrely at 30k feet...what I did wrong?

I was trying an MP mission where the P-51 side is supposed to escort bombers to a target. Of course, they're cruising at 200 indicated and I'm trying to keep up while climbing up to meet them at angels 30, which takes me the entire transit to the AO.

I met with them just as they got jumped by 109's and 190's, so I turned to meet the bandits. I had assumed that I'd outperform the German fighters up this high. However, I found that every input I gave the aircraft sent it into a spin or at least a one-wing stall. Dogfighting was out of the question (and of course AI planes suffer no ill effects from altitude, damage, energy loss, etc...ever) and the German planes zoomed around like nobody's business, carved up the bombers and then took shots at me before I broke away.

What's up? I thought the Mustang should have advantages up at 30k, but instead found that it was extremely unstable, uncontrollable and totally at the mercy of the AI fighters. I tried using "silk hands" to dampen the effects of the violent loss of control, but this instead left me flying almost in a straight line and thus an easy target.

There has to be something I'm misunderstanding.

That is precisely why I use curves on my joystick. When you are higher up density of air is lower and u need better lift to stay in the fight. You will stall at higher IAS.

AI may be broken but I have fought human 109s there. If u believed some on this forum in Mustang's high alt superiority, that is not going to happen. They are pretty equal there.

High up you have to do large maneuvers, keeping up speed, watch your AoA and be gentile. You might have an edge in turn, but it is exploitable only with very good rudder coordination. It doesn't become a difference between day and night there.

I have to investigate the speed at that alittude though, it realy does seem sluggish.

If I can relate with what I have learned in Jets (and other simulators with props) about the high-alt modelling, I share some pointers.

- Accelerate while still low-medium altitude. If you climb the fastest way up and then try to level out and accelerate you can forget it, thrust is so low up there that you will struggle to get to the speeds needed to maintain a decent performance.

- Service ceiling doesn´t mean that the aircraft will be confortable in anything below that.

- Keep your speed high, you need the lift.

- Relax on the stick inputs, stall speeds are amplified my a large margin.

- Fighting against AI up there is harsh because as you´ve noticed, they just use SFM and won´t care about your struggle. Players will have a different experience.

And why are the bombers flying at 30000 feet? I thought B-17s typically operate somewhere under 20k with bombs and fuel? That´s a much more reasonable altitude for you to fight in aswell. Maybe the mission designer was just being a capital D.

The bombers were cruising at 30k but dropped to 25k for the bombing run.

Also, any advice on prop RPM? I was keeping 2700 but am curious if a lower RPM/pitch might take a bigger bite of air per revolution. Any validity to this at high altitude?

I was keeping 2700 but am curious if a lower RPM/pitch might take a bigger bite of air per revolution. Any validity to this at high altitude?

Lower RPM means higher pitch, not lower. There is absolutely no validity to this at all.

Thrust is first and foremost dictated by engine power output and your engine craves RPM to drive the supercharger at high enough RPM so it supplies enough boost and the engine itself will provide more power also.

The RPM governor is there to optimally set the pitch for your power output and airspeed so you don't have to. Worry about your engine life and power output, not your prop pitch.

Actually the governor adjusts pitch to try and keep the rpm at the speed that you select on the tachometer with the prop control. Whether it is optimal or not for the task at hand depends upon the pilot.

Do people adjust fuel mixture in the say the P-51 at high alts?

If that's possible ???

Keep rpm on full ???? (all the way forward )

They don't, and they can't, because automatic mixture regulator is responsible for that. That's why You leave mixture lever at "Run" and forget about it, unless emergency situation happens.

You wouldn't even be able to climb to very high altitudes with the lever at "Emergency Rich", as numerous forum threads from novice P-51 flyers showed here a few times (engine flooding).

That is precisely why I use curves on my joystick. When you are higher up density of air is lower and u need better lift to stay in the fight. You will stall at higher IAS.

 

Don't you mean a high true airspeed? Indicated is your best indicator of what the wing is experiencing.

Don't you mean a high true airspeed? Indicated is your best indicator of what the wing is experiencing.

S!

Actually the governor adjusts pitch to try and keep the rpm at the speed that you select on the tachometer with the prop control. Whether it is optimal or not for the task at hand depends upon the pilot.

That's sort of my point.

Negative, he means IAS. IAS change with altitude, Vs, Vx, Vy, Vne, all of them rises.

 

S!

Yes, but stall always occurs at same IAS regardless of altitude.

Yes, but stall always occurs at same IAS regardless of altitude.

To be more precise, stall has nothing to do with airspeed, IAS, CAS, EAS, TAS or GS, it depends exclusively on the angle of attack.

S!

Why should that be the case in your opinion?

S!

I mean the aircraft handbook, sorry. Don't remember but IIRC C152 handbook for instance told a change about 2 to 5 knots up to 5.000ft (from my head, but something like that), may be 7 to 10 knots up to 10.000ft (but I would have to take a look at it). It's a small difference, the same as Vx and Vy changes from 63 and 67 KIAS to 65 and 70KIAS at 5000ft, a so small difference we usually use 65 and 70 since take off (I don't think even Chuck Yeager could hold exactly 63 or 67KIAS since take off all the way up to 5.000 ft with a glued anemometer :) ).

P-51 has higher power thus can operate to altitudes and speeds quite higher. But you can't climb 30.000ft and expect Vs and Vy to be the same. Wasn't operating speeds stated in P-51 handbook? It's a long time since last time I read.

S!

Checking a quick chart I found at hand I mistook Vx and Vy change, better climb speed decrease with altitude, sorry. So for instance SL Vy would be 67 KIAS, 64 KIAS @ 5K ft and 61 KIAS @ 10K ft in a C152. Still we used 70, 65 and so usually as easiest to read and keep in anemometer.

S!

We crossed messages.

approximately the same in terms of IAS.

S!

Sorry mates, you're right. Stall happens at higher IAS at higher altitudes, so you reach critical AoA sooner, or easier in lower atmospheric densities :smilewink:. Stall doesn't happens at the very same IAS regardless of altitude, read the handbooks mates.

 

S!

No amount of referring to handbooks can change the truth, sorry buddy.

From

https://en.wikipedia.org/wiki/Indicated_airspeed

[...]the aircraft behaves similarly at the same IAS no matter what the TAS is[...]

[...]IAS closely reflects dynamic pressure[...]

[...]at typical civilian operating speeds, the aircraft's aerodynamic structure responds to dynamic pressure alone, and the aircraft will perform the same when at the same dynamic pressure[...]

Yepp, it's pretty basic. Vmin, best X/Y speed, etc...these don't change with altitude in IAS. They do occur at a higher TAS, but your ASI will show the same IAS as at sea level.

No amount of referring to handbooks can change the truth, sorry buddy.

Yepp, it's pretty basic. Vmin, best X/Y speed, etc...these don't change with altitude in IAS. They do occur at a higher TAS, but your ASI will show the same IAS as at sea level.

S!

Although it's a hard to find info a quick google search thrown some things. They're talking about airliners at high cruise altitudes but still applies,

Folks, I hate to disagree with all you guys, but this issue is pretty important so I think we need to correct this before MORE people get incorrect information.

 

Stall speed INCREASES with altitude. Whether you look at IAS, CAS, EAS or TAS, they ALL increase with altitude.

 

Since PFDs display CAS (not IAS), and TAS has no importance whatsoever in the stall margin, we should only really talk about CAS and EAS.

 

EAS is really the airspeed that we need to be concerned about when relating airspeed to stall margin, and the stall speed in EAS increases with altitude.

A good rule of thumb is the 2kts/5000ft rule, but at cruise altitudes the increase is even greater than this approximation and varies depending on aircraft type.

The CAS stall speed will increase by an even greater degree due to compressibility effects... and since this is what we see on the PFD, we should keep and even greater "buffer".

 

 

I urge everyone to review some good aerodynamics or performance books because IMHO there should be no confusion whosoever over such a basic and fundamental aerodynamic principle. High altitude aerodynamics must be one of the most poorly understood areas of flight training and unfortunately it is not taught enough (FAA requires it for a commercial but applicants are seldom quizzed on it....and my experience is that regional airlines do a very poor job at teaching it).

Unfortunately, there are WAY to many books that contain incorrect or in some cases outdated information, so be careful about what you trust.

 

Long story short -> if you ever find yourself having to deviate from company profile at high altitudes, remember stall speed will be higher than what you would otherwise encounter at sea level.

Again, I hate to disagree but this is simply not true. Stall speed increases with altitude, NO MATTER WHAT. I am not talking simply about a airspeed measurement error (like the fact that at altitude your PFD shows CAS and therefore you are reading a higher value than your plane's EAS, which again is what is related to stall margin)... I am saying that aerodynamically the wing will stall at a higher airspeed, regardless of airspeed measurement errors. To put it differently, it will stall at a lower CLmax.

 

This change is stall speed is SIGNIFICANT. Like I wrote on my previous post, we're looking at least at a 2kt/5000ft, but likely more depending on wing design. So at 40,000' your stall speed is at least 16kt higher than what it would be at sea-level, but most likely EVEN MORE than that due to additional effects that are wing-specific.

 

I hate to be disagreeing with everyone, but we've had so many cases of crews flirting with the low-end of the envelope at high altitude in the last 10 yrs (some fatal) that I feel there should be no doubts about the effect that altitude has on stall speed. Like rickair said, if you stick with the HIGH ALTITUDE values on your speed cards you should be safe (not to be confused with landing V'ref speeds....those are calculated for sea level).

 

I will try to get exact numbers for different airplane types, but the 2kt/5000' rule of thumb should apply to all airplanes.

BINGO!

higher altitude -> lower air density + lower air viscosity (because of lower temps) -> lower Reynolds number -> less kinematic energy in the boundary layer to oppose adverse pressure gradients -> earlier flow separation -> lower CLmax and AOAcrit -> higher stall speed

 

Thats the 2kt/5000ft... in addition to this effect, there is also the effect of local supersonic flow on the wing upper surface that prevents you from truly reaching the sea-level-CLmax equivalent. Even at speeds below Mcrit, at high AOA the pressure distribution over the upper surface causes the boundary layer to accelerate around the leading edge, and at high enough CL (and thus large enough pressure gradients) the flow can be accelerated to locally supersonic values. Of course the magnitude of this effect will depend on wing design, in addition to Mach, altitude and temperature.

 

As for sources, I remember seeing the 2kt/5000ft in various books, but after looking for the last hour I can only find it in my student manual from the Naval Test Pilot School, although if you ask me, I trust this manual more than certain aero books... I found a PDF version here http://www.usntpsalumni.org/USNTPS_FTM_108.pdf (look for page 3-27)

 

As for the reduction of CLmax due to Mach you can see a chart here Structural loads analysis for ... - Google Books that shows a significant reduction of CLmax at Mach speeds as low as .2-.4

This book also has a chart on page 263 that shows the slows speed stall speed increasing with increasing altitude. But I hope by now everyone in happy with this concept. If you really wanted, really you could just take any BOB chart for a transport category airplane (buffet onset boundary) and you'll see the same relationship there (look for the slow-speed buffet).

 

Another worthwhile read is Aircraft Performance - Google Books chapter 2.6

 

Bottom line is that the only TRUE way to calculate stall speed at altitude for a given wing is through flight testing. And even then, as ryan's EXCELLENT FSI article pointed out, it's not as easy as it seems. There are so many dynamic factors that play in the determination of stall speed that it's not easy to isolate all the variables.

http://www.airlinepilotforums.com/636145-post7.html

http://www.airlinepilotforums.com/636490-post11.html

http://www.airlinepilotforums.com/636693-post18.html

S!

P.D.: still you could read the handbook or make the google search for yourself :P :smilewink:.

P.D.: sorry mates, but you were making me doubt as my memory isn't as good as used to be, but I swear I heard my instructor talking about this even I'm not an ATPL. Luckily I'm not going crazy :lol:.

No amount of referring to handbooks can change the truth, sorry buddy.

 

From

https://en.wikipedia.org/wiki/Indicated_airspeed

 

furthermore

 

and finally

Now please, show me a real handbook that condradicts this. Just saying "You're wrong because handbook" doesn't cut it.

An aircraft performs the same at a given IAS at any altitude, but only if it remains below critical AoA. Critical AoA reduces with altitude so stall occurs at higher IAS.

There's no contradiction here, even though it seems like there is on first inspection.

Again, optimum climb speed decreases with altitude.

Vmin means a lot because it's related to stall speed, in case of the P-51 +25%. So stall speed can't increase at the same time.

Vmin Minimum speed for instrument flight (IFR) for helicopters

https://en.wikipedia.org/wiki/V_speeds

In short, stall speed changes with altitude, that's the point. Already said it's a small change. Still it does change :P .

S!

I'm sorry that I wrote Vmin as an example as the P-51 manual calls this 'minimum safe gliding speed'. Don't know if an 'official' V speed exists for this...

S!